Your problem isn't resolution but quality.
First of all set your render resolution the same as your movie resolution (currently your render resolution is 551x367 while your movie resolution is 720x480). Most people render out as separate pictures with the best quality and use another program to turn these into a movie. Most separate programs are capable of providing a better quality (with many different formats and often better compression). All Poser movie compression algorithms are only good enough for a quick preview, they do not provide enough capabilities to properly create good quality movies.
Also keep in mind that with moving pictures quality of single frames is less of an issue. As always it is a compromise between quality and size, the better the quality of the individual frames the bigger the movie file becomes.